During core sampling/drilling (typically for mineral exploration), a drilling apparatus with a high speed drill bit is used. During this process, the drilling apparatus rotates thin walled drill rods (forming a casing) from surface at high speed, often at greater than 1000 rpm. At the distal end of the drill rods/casing is a (usually diamond) core drill bit—which has a hollow centre. As the drill bit is rotated and pushed forward into the formation being drilled, the core sample moves into an annulus above the drill bit known as a core (catcher) barrel. Typically, a core barrel is 1.5-6 metres long.
Once the drill bit has advanced into the substrate sufficiently for the core barrel to be full, the drilling stops. From surface, a wire cable and overshot is lowered down through the drill casing until the overshot attaches to the core barrel (and associated components). The wireline is then retracted to surface pulling the core barrel and core sample in the barrel (which is retained by a snap ring or similar). The core can then be removed from the core barrel for analyses, while the drill rods and drill bit remain in the ground acting as a temporary casing for the bore.
While core sampling/drilling using diamond (or other) drill bits is the industry standard for taking rock samples, there are problems. One such problem is that the core sample will often break and block the core barrel. This means that when the wireline is raised to surface for the inner assembly (core barrel, core sample swivel, latching system etc.), it transpires that the core barrel is only partially full (at best), or in fact the rock core has wedged in such a way as to stop further advancement of the drilling system. Also, core drilling using diamond drill bits is slow and expensive, with the core being recovered often at a rate of 20 metres or less per 12 hour shift. In extremely hard formations the drilling may cease.